Production technology and nitriding treatment of seamless carbon steel pipes

Seamless carbon steel pipes have hollow cross-sections and are widely utilized for conveying fluids, such as in pipelines transporting oil, natural gas, coal gas, water, and certain solid materials. Based on their cross-sectional shape, seamless carbon steel pipes can be divided into round pipes and special-shaped pipes. Among these, round pipes are most commonly used because, for a given circumference, the circular shape has the largest area, allowing more fluid to be transported. Additionally, round pipes distribute internal or external radial pressure more uniformly, making them the preferred choice in most applications.

Seamless carbon steel pipes serve various industries and applications, such as petroleum geological drilling, cracking pipes in petrochemical industries, boiler pipes, bearing pipes, and high-precision structural steel pipes used in automobiles, tractors, and aviation.

Overview of the Production Process of Seamless Carbon Steel Pipe:

- Cold-Rolled (Drawn) Seamless Carbon Steel Pipe Process:

Billet preparation → Pickling and lubrication → Cold rolling (drawing) → Heat treatment → Straightening → Finishing → Inspection

- Hot-Rolled Seamless Carbon Steel Pipe Process (with Main Inspection Stages):

Tube blank preparation and inspection → Tube blank heating → Perforation → Tube rolling → Tube reheating → Sizing (reducing) → Heat treatment → Finished tube straightening → Finishing → Inspection (non-destructive, physical and chemical, bench inspection) → Storage

Nitriding Treatment of Seamless Carbon Steel Pipe:

- Nitriding Time:

During nitriding, after a specific period, the surface hardness of the carbon steel pipe reaches its maximum value. If nitriding continues beyond this point, the hardness begins to slightly decrease. The time to reach peak hardness is shorter at higher nitriding temperatures, though the overall hardness value is lower. The growth of the "K layer" (compound layer) increases as nitriding time progresses.

- Decomposition Rate of Ammonia:

The decomposition rate of ammonia refers to the percentage of hydrogen and nitrogen produced by the decomposition of ammonia within the furnace gas volume. A high decomposition rate may cause hydrogen in the furnace to inhibit the penetration of nitrogen atoms, resulting in nitrogen stagnation. Factors such as furnace pressure, ammonia flow rate, surface condition of the seamless carbon steel pipe, and the presence or absence of catalysts influence the decomposition rate. For optimal results, the decomposition rate is typically controlled between 18% to 45%.

- Nitriding Temperature:

At a nitriding temperature of 500°C, the surface hardness of the carbon steel pipe is at its highest. Exceeding this temperature causes the hardness to decrease. It is important to consider the relationship between nitriding temperature, surface hardness, nitriding layer thickness, and potential deformation of the seamless carbon steel pipe when determining the best temperature for nitriding.